Fuel cells are highly efficient, silent, power generating systems in which a fuel undergoes an oxidative chemical reaction and generates electrical current. Fuel cells are coming into increasing use as power sources for a variety of fixed and mobile applications: including transportation systems; electronic devices: including computers, cellular phones, and specialized electronic equipment; and the like. Fuel cells are also being used as stationery power sources for communication installations, off-grid building power, and as backup power sources.
In a fuel cell, an oxidative reaction, typically oxidation of a fuel, occurs at a catalytic surface of a first electrode, and a reduction reaction, typically reduction of oxygen, occurs at a second electrode surface. The electrodes are separated, as for example, by a membrane which is ion permeable but has very low electron conductivity. In a cell of this type, an external circuit allows for the passage of electrons from one electrode to the other so as to balance the chemical reactions, and it is this flow of electrons which generates power. For reasons of simplicity, and material compatibility, most fuel cells presently in use utilize hydrogen as a fuel gas and air or oxygen as an oxidizer. Storage and delivery of the hydrogen fuel can complicate and limit the utility of such fuel cells. In order to overcome these problems, significant efforts have been extended to develop fuel cells running on liquid fuels, and methanol-fueled fuel cells, referred to also as direct methanol fuel cells (DMFC), have been developed.
In conventional direct methanol fuel cells of the type known in the art, methanol is oxidized at an anode, which is typically comprised of a body of electrically conductive material having a platinum-based catalyst thereupon. The anode reaction is as follows:CH3OH+H2O→CO2+6H++6e− 0.02VIn a cell of this type, the cathode reaction is as follows: 3/2O2+6e−+6H+→3H2O 1.23V
In a cell of this type, the anode and cathode are separated by an ion exchange membrane which allows protons generated in the anode reaction to pass therethrough to the cathode. The membrane does not permit the passage of electrons, and these flow through an external circuit to provide an output voltage. Membranes of the type typically used in such cells can include perfluorosulfonate materials such as those sold under the trademark Nafion by the DuPont Chemical Corporation. The theoretical potential difference in a cell in which the foregoing reactions take place is 1.21V. However, the practical cell voltage of such fuel cells is only 0.7V under open circuit conditions and 0.4V under typical operational discharge rates. This is because of the slow catalytic kinetic rate of methanol oxidation at the anode, and methanol cross-over from the anode to the cathode through the electrolyte membrane, which causes cathode-electrode depolarization. These phenomena are well known in the art.
From the foregoing, it will be appreciated that there is significant room for improvement in the field of direct methanol fuel cells. Any improvement which can increase the voltage of such cells will greatly enhance their utility by allowing more compact power sources to be implemented. As will be described in detail hereinbelow, the present invention concerns membrane assemblies for fuel cells which may be advantageously incorporated into a variety of fuel cells, including direct methanol (or other liquid fuel) fuel cells. The direct methanol fuel cells incorporating the assembly of the present invention are reliable, rugged, and operate at high-output voltages. These and other advantages will be apparent from the drawings, discussion and description which follow.